Ultrasonic flaw detectors can detect internal defects of specimens without destruction of the specimens and are employed in many fields. The existence or non-existence of a defect inside a specimen is checked over a predetermined area of the specimen in many instances. In this case, the inspection is conducted by scanning the above-described area of the specimen with ultrasonic waves radiated from a probe. Actually employed as such a probe includes an array probe constructed of many piezoelectric elements arranged in a line. An ultrasonic flaw detector making use of such an array probe will hereinafter be described.
FIG. 1 is a perspective view of a scanner unit of the conventional ultrasonic flaw detector. FIGS. 2(a) and 2(b) are plan and side views of an array probe, respectively. In each of the drawings, there are shown a water tank 1 for inspection, water 2 charged in the water tank 1, and a specimen 3 placed on the bottom wall of the water tank 1. Designated at numeral 4 is a scanner, which is constructed of the following members: a scanner table 5 on which the water tank 1 is mounted, frames 6 fixed on the scanner table 5, an arm 7 extending between the frames 6, a holder 8 mounted on the arm 7, a pole 9 pendant from the holder 8, and an array probe 10. The frames 6 can move the arm 7 in the direction of Y-axis by an unillustrated mechanism, while the arm 7 can move the holder 8 in the direction of Y-axis by a mechanism which is free of illustration. Further, the holder 8 can move the array probe 10 in the direction of Z-axis (the direction perpendicular to X-axis and also to Y-axis) in association with the pole 9 by means of a mechanism (not shown).
The array probe 10 is constructed of a number of minute piezoelectric elements (hereinafter called "array elements") arranged in a line. The direction of arrangement of the array elements is in conformance with the direction of X-axis. Whenever a pulse is applied, each array element radiates an ultrasonic wave and then converts a reflected wave of the ultrasonic wave by the specimen 3 to a corresponding electrical signal. The individual array elements are indicated by numerals 10.sub.1 -10.sub.n in FIGS. 2(a) and 2(b), in which dots indicate points of sampling. Symbol YP indicates the sampling pitch in the direction of Y-axis, while symbol XP represents the sampling pitch in the direction of X-axis. In addition, symbol AP shows the pitch between the adjacent array elements 10.sub.1 -10.sub.n. Designated at numeral 11 is a casing in which the array probe 10, etc. are accommodated.
Here, the function of the array probe 10 shown in each of the above drawings is described in brief with reference to FIGS. 3(a) and 3(b). In FIG. 3(a), there are illustrated array elements T.sub.1 -T.sub.9 arranged in a line, delay elements D.sub.1 -D.sub.9 connected to the respective array elements T.sub.1 -T.sub.9, and pulses p to be inputted to the respective array elements T.sub.1 -T.sub.9. The delay elements D.sub.1,D.sub.9 have been set at the same delay time (t.sub.19). Likewise, the delay elements D.sub.2,D.sub.8 at the same delay time (t.sub.28), the delay elements D.sub.3,D.sub.7 at the same delay time (t.sub.37), and the delay elements D.sub.4,D.sub.6 at the same delay time (t.sub.46). The relationship among the delay times thus set can be expressed by the following inequality: EQU t.sub.19 &lt;t.sub.28 &lt;t.sub.37 &lt;t.sub.46 &lt;t.sub.5 ( 1)
where t.sub.5 stands for the delay time of the delay element D.sub.5.
Now, the delay times of the individual delay elements D.sub.1 -.sub.9 are set at desired values while maintaining the relationship of the inequality (1), and the pulses p are inputted. From the array elements T.sub.1 -T.sub.9, ultrasonic waves are then radiated in accordance with the delay times so set, i.e., first from the array elements T.sub.1,T.sub.9 and last from the array element T.sub.5. The ultrasonic waves radiated in the above manner then advance radially and inwardly, so that there is a point where the maximum amplitudes of oscillations of the ultrasonic waves radiated from the respective array elements all coincide. This point is indicated by letter F in FIG. 3(a). Since the magnitude of the resulting ultrasonic wave is far greater at the point F compared to that of ultrasonic wave at any other point, the ultrasonic waves from the respective array elements T.sub.1 -T.sub.9 become as if converged at the point F as indicated by dashed lines. In other words, the application of suitable delays to the radiation of ultrasonic waves from respective array elements arranged in a line can develop a situation similar to the convergence of such ultrasonic waves at the point F. This point F will therefore be called "the point of convergence". Described further, the array elements T.sub.1 -T.sub.9 outputs an ultrasonic beam B which converges at the point F of convergence as indicated by the dashed lines. If the respective delay times are set shorter than the above-described delay times while maintaining the relationship of the inequality (1), the point F of convergence is shifted to a farther point F' of convergence as indicated by alternate long and short dash lines (beam B'). It is therefore possible to select the position of the point of convergence by adjusting the delay times of the individual delay elements D.sub.1 -D.sub.9. When used for the inspection of the specimen 3, the depth of the point of inspection can be selected.
FIG. 3(b) is a schematic illustration of the function of the array probe 10 shown in FIGS. 2(a) and 2(b). In the drawing, numerals 10.sub.1 -10.sub.n indicate the same array elements as depicted in FIG. 2(a). Delay elements are connected to the individual array elements 10.sub.1 -10.sub.n although not illustrated. In the illustrated example, m pieces of array elements 10.sub.1 -10.sub.m are first selected and the delay times of ultrasonic waves to be radiated from the array elements are set appropriately, whereby the ultrasonic waves are apparently caused to converge at one point of convergence as described above. In FIG. 3(b), this point of convergence and an apparent ultrasonic beam are indicated by symbols F.sub.1 and B.sub.1, respectively. Next, the vibration of array elements is shifted by one element so that delay times of the same pattern as the delay times applied to the array elements 10.sub.1 -10.sub.m in the preceding vibration are applied to the m pieces of array elements 10.sub.2 -10.sub.m+1. For this vibration, the point of convergence is indicated by symbol F.sub.2 while the resulting ultrasonic beam is designated by symbol B.sub.2. Thereafter, the vibration of array elements is shifted successively one by one. The array elements 10.sub.n-m+1 -10.sub.n are finally selected, to which delay times of the same pattern are applied to obtain a point F.sub.n-m+1 of convergence and an ultrasonic beam B.sub.n-m+1. As a consequence, ultrasonic scanning can be performed from the point F.sub.1 of convergence to the point F.sub.n-m+1 of convergence by the array probe 10 in the manner described above. The scanning will hereinafter be called "electronic scanning" as it is electronically performed at a high speed. In FIG. 3(b), AP and SP indicate the pitch of the array elements and the sampling pitch, respectively. They are equal to each other in the illustrated example.
A description will next be made of a control unit of the ultrasonic flaw detector making use of the array probe described above. In this description, assume by way of example that an area of a specimen, said area being to be inspected, has a length of 120 mm in the direction of X-axis, the array probe 10 is equipped with 128 array elements, the pitch of the array elements is 1 mm, eight array elements are vibrated at the same time, and the scanning of the specimen is performed with 121 beams in the direction of X-axis. FIG 4 shows the arrangement of the above array elements, in which there are illustrated the array probe 10, array elements 10.sub.1 -10.sub.128 and ultrasonic beams B.sub.1 -B.sub.121. The individual array elements 10.sub.1 -10.sub.128 are indicated by numerals 1-128 in the drawing. The control unit is indicated at numeral 11.
FIG. 5 is a block diagram of the control unit 11 shown in FIG. 4. In the drawing, numeral 10 indicates the array probe. Designated at numeral 12 is a microprocessor, while numeral 13 indicates a transmission delay circuit for delaying, by predetermined times, vibration of the individual array elements which are adapted to give off ultrasonic beams B.sub.1 -B.sub.121, respectively. Only one transmission delay circuit 13 is provided for the individual ultrasonic beams B.sub.1 -B.sub.121. There are also illustrated a matrix circuit 14 and a distributor 15. They are provided to use the transmission delay circuit 13 commonly for the respective ultrasonic beams B.sub.1 -B.sub.121. Designated at numeral 16 is a transmit-receive circuit, which outputs vibrating pulses to the individual array elements 10.sub.1 -10.sub.128 of the array probe 10 and also receives signals of reflection waves from the individual array elements 10.sub.1 -10.sub.128. The constructions of the matrix circuit 14, distributor 15 and transmit-receive circuit 16 will be described in further detail with reference to FIG. 6, FIG. 7 and FIG. 8. Designated at numeral 17 is a shift register, which serves to successively connect the transmit-receive circuit 16 to groups of eight array elements, each group being to be employed to form a single ultrasonic beam. Numeral 18 indicates an adder having the same construction as the distributor 15 except that the input and output are opposite, and numeral 19 designates a matrix circuit of the same construction as the matrix circuit 14. Designated at numeral 20 is a waveform adder, which brings into coincidence the phases of eight signals outputted from the matrix circuit, followed by the addition. Each output of the waveform adder 20 is processed suitably and then displayed in a desired mode. Based on the output thus displayed, it is determined whether the specimen contains a defect or not.
The operation of the control circuit 11 will next be described with successive reference to FIG. 6, FIG. 7 and FIG. 8.